Electrographic imaging and development processes have been extensively described in both the patent and other literature. Generally, these processes include the common steps of forming an electrostatic charge image, often called an electrostatic latent image, on an insulating surface, such as a photoconductive insulating layer coated on a conductive support. The electrostatic latent image is then rendered visible by a development step in which the charge image-bearing surface is brought into contact with a suitable elctrostatic image developer composition which deposits toner particles on either the charge or uncharged image areas. After development, the visible image is either fixed directly to the electrostatic charge-bearing surface or it is transferred to another surface, such as paper, where it is there fixed.
Liquid developer compositions of the type described, for example, in Metcalfe et al, U.S. Pat. No. 2,907,674, issued Oct. 6, 1959 have been used to develop latent electrostatic images. Such developers usually comprise a stable dispersion of charged particles, known as toner particles, comprising a pigment, such as carbon black, generally associated with the resinous binder, such as, an alkyd resin, dispersed in an electrically insulating liquid which serves as a carrier. A charge control agent is sometimes included to stabilize the magnitude and polarity of the charge on the toner particles. In some cases, the binder itself serves as a charge control agent.
U.S. Pat. No. 3,939,087 and No. 4,019,911 issued to Vijayendran et al recognize high humidity as a problem in obtaining good prints in electrophotography. These patents also teach the addition of highly hydrophobic agents, such as silane treated fumed silica to the liquid developer to help solve this problem as the silanes are not sensitive to humidity.
U.S. Pat. No. 4,058,470 issued to Moschovis et al also teaches the addition of hydrophobic colloidal silica to a liquid developer to increase the oleophilicity of the liquid developer.
Mayer U.S. Pat. No. 2,877,133 and No. 2,890,174 teach a liquid developer having silica aerogel mixed therein as a suspending agent for the opaque marking particles.
While high humidity conditions are detrimental to liquid electrographic developers generally in that the density of the developed image is decreased, more serious problems can be encountered as the developers become more complex. For example, in U.S. Pat. No. 3,788,995 issued to Stahly et al copolymers are described that stabilize both the dispersion of the toner particles in the carrier liquid and the charge. These copolymers contain hydrophilic polar groups which can ionize in the presence of ambient moisture to form ionic species that compete with charge toner particles for latent image sites, thus reducing image density.
In addition, certain colorants when added to liquid developers to impart a desired color to a finally developed image can increase the difficulties with regard to image densities under high humidity conditions. It is believed that these colorants contain a large number of hydrophilic groups which, increases the hydrophilicity of the medium in the presence of water, thus causing increased ionization of the hydrophilic polar groups present in the copolymers as previously discussed.
Regardless of the mechanism by which degradation of the finally developed image occurs, it is essential that the particular developer employed produce uniform images over extremely broad relative humidity conditions.